This invention relates generally to radio communication systems and more particularly to apparatus and methods used to test such systems.
As is known in the art, radio communication systems are being used extensively today. Such systems enable communications among fixed and mobile telecommunication subscribers. Information is sometimes transmitted through a network of wireless radio communication stations. Pairs of such stations are in bi-directional communication with each other. Thus, each station typically includes a transmitter, adapted to transmit information to another station, and a receiver, for receiving information from the other station. In order to isolate the signal transmitted by a station from the receiver at such station, the transmitter at the station transmits at a carrier frequency, f.sub.TL, which differs in frequency by a fixed offset frequency, f.sub.os, from the carrier frequency, f.sub.TR, transmitted to such station by a remote, or "far end" station. Therefore, the receiver at the local station is tuned to a different frequency, f.sub.R =f.sub.TR (i.e., to the carrier frequency transmitted by the "far end" station) from the carrier frequency, f.sub.TL =f.sub.R -f.sub.os, transmitted at the local station.
As is also known, the local oscillators used in the transmitter and receiver at a station are adapted to have the frequency produced by them (i.e., the carrier frequency, f.sub.T, produced by the transmitter and the local oscillator used to tune the receiver to the carrier frequency, f.sub.R =f.sub.TR of the "far end" station) selected with in a range of frequencies, typically in increments of 10's of KHz over several 1 Ghz. The frequency control data used to select the carrier frequencies f.sub.T, f.sub.R is typically provided by a microprocessor at the local station. Once selected, the carrier frequencies f.sub.T, f.sub.R usually remain fixed during normal operation of the system. Such microprocessor generated frequency control data is used to fine tune the receiver so that signal received by the local station from the "far end" station are heterodyned to the predetermined intermediate frequency, f.sub.IF =f.sub.TL -f.sub.R, of the receiver.
As is also known, it is sometimes necessary to test the operation of a local station's transmitter and receiver. One such test is used to verify the local station's transmitter's carrier frequency. One technique used to perform such test is to transmit a test signal from the local station to the "far end" station and have the "far end" station retransmit it back to the receiver of the local station. Such test is sometimes referred to as a "far end loop-back" test. While such test may provide some information about the operation of the two stations, it does not allow one to isolate the source of any fault; i.e., if there is a detected fault, one is not able to isolate it as a fault in the local station's transmitter or a fault in the operation of the "far end" station doing the re-transmission. Another test suggested is to provide a two input port-single output port switch. During normal operation, one of the pair of input ports would be coupled to the receiver output port of the station's transmit/receive (T/R) switch. During a test mode, the other one of the pair of input port would be serially coupled to a frequency converter having a local oscillator producing the fixed offset frequency, f.sub.os. The signal at the output of the third port is thereby shifted in frequency to the station's transmitter carrier frequency f.sub.TL. In this way, the local station's receiver would be tuned to the frequency of the local station's transmitter. The signal produced at the output of the test mode tuned receiver could then be used to detect faults in the local station transmitter. While such technique eliminates faults which may be at the far end site, it requires additional equipment, i.e., a switch and frequency converter.